The semiconductor industry uses different types of semiconductor-based image sensors, including charge coupled devices (CCDs), photodiode arrays, charge injection devices (CIDs), hybrid focal plane arrays, and complementary metal oxide semiconductor (CMOS) image sensors. Current applications of such image sensors include cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detector systems, image stabilization systems, and other image acquisition and processing systems.
Semiconductor image sensors include an array of pixel cells. Each pixel cell contains a photo-conversion device for converting incident light to an electrical signal. The electrical signals produced by the array of photo-conversion devices are processed to render a digital image. The amount of charge generated by the photo-conversion device corresponds to the intensity of light impinging on the photo-conversion device. Accordingly, it is important that all of the light directed to a photo-conversion device impinges on the photo-conversion device rather than being reflected or refracted toward another photo-conversion device, which would produce optical crosstalk.
For example, optical crosstalk may exist between neighboring photo-conversion devices in a pixel array. Ideally, all incident photons on a pixel cell are directed towards the photo-conversion device corresponding to that pixel cell. In reality, some of the photons are refracted and reach adjacent photo-conversion devices producing optical crosstalk.
Optical crosstalk can bring about undesirable results in the images produced by imaging devices. The undesirable results can become more pronounced as the density of pixel cells in image sensors increases and as pixel cell size correspondingly decreases. Optical crosstalk can cause a blurring or reduction in contrast in images produced by the imaging device. Optical crosstalk can also degrade the spatial resolution, reduce overall sensitivity, cause color mixing, and lead to image noise after color correction.
FIGS. 1A and 1B are schematic cross-sectional views of a portion of an optical waveguide aimed at reducing optical crosstalk, which has an optically transparent core 10 surrounded by, or enclosed within, a cladding structure 20. The cladding structure 20 has a lower refractive index than the refractive index of the core 10. Light introduced into an end of the core 10 undergoes total internal reflection at the refractive boundary, causing the light to be guided along an axis of the core 10. Total internal reflection, however, occurs only when the angle of incidence θ (FIG. 1A) is larger than a critical angle θc. The angle of incidence θ is measured with respect to the normal at the refractive boundary. If light strikes the refractive boundary at an angle θ (FIG. 1B) smaller than the critical angle θc, optical leakage and power loss occur resulting in optical crosstalk and reduced quantum efficiency, which are undesirable. Accordingly, there is a need and desire for an improved method and structure for reducing optical crosstalk in imaging sensors.